The present invention relates to signal detection means which enables information, such as utility usage, to be directed along telephone lines when an interrogation signal is received on the same lines. More particularly, the invention disclosed herein pertains to a very low power consuming signal detection device which is attached to a customer's telephone lines. The device senses a signal directed to it from a central office while in the on-hook (low current) condition and activates other circuitry, such as a meter reading apparatus, to operate in the off-hook (high current) condition. The terms "on-hook" and "off-hook" are commonly used in the telephone industry to describe the operating condition of a telephone or other device connected to a telephone line. In the normal state, the handset is on-hook and the switch disconnects the telephone from the line. In this open circuit condition, no current flows from the battery supply at the telephone central office to the telephone. However, when the receiver is lifted off-hook, the telephone is connected to the line and current flows.
Over the past two decades numerous automatic meter reading systems have been innovated. Such systems rely on radio, telephone, cable TV, fiber optics, power lines and even free space optical communication to transfer data from a remote source to a central point for processing. Although a number of systems have proven technically feasible, the high cost of such systems has precluded widescale acceptance in the marketplace. More recently, automatic meter reading systems which utilize existing telephone networks to access individual utility meters have proven cost effective for the first time. Such meter reading systems have many important advantages. First, the telephone lines are already in place and are juxtaposed to approximately 96% of all the utility meters in the United States. Second, utilization of telephone lines for modem to modem communications between computers has long proven a very reliable means of data transfer. Third, all telephone lines are connected to a central battery source which can provide small amounts of power to communication electronics located at the customer's premise.
A number of prior art patents have disclosed systems which utilize non-dedicated telephone lines as a means to access devices for reading meters at the customer premises from a central location. One such patent is Willis et al., U.S. Pat. Ser. No. 4,394,540, which discloses a meter reading device for reading and transmitting meter data in response to a call from the central office. This device isolates the customer's telephone for a period of time each day, at which time it intercepts each call to determine if it is a request for meter data. The device answers the call, sends a signal back to the central office, and awaits a confirmatory signal. If the call is not for data, the device artificially generates a ring on the customer's telephone. Thus, the system delays and perhaps masks normal calls to the subscriber. This system is unduly complex and expensive for broadscale development in an automatic meter reading system and requires that a clock in each unit be coordinated with the central computer. Also, the time delays involved in accessing data in such a manner may be practically and financially disadvantageous. U.S. Pat. Ser. No. 4,654,868 to Shelley is similar to Willis et al. in that it intercepts the first ring to the customer's telephone. However, in Shelley the system is always operable. If, and only if, a single ring is transmitted to the system, it calls back the central computer to send data. The need for a telephone dialing device and other circuit components in each customer unit adds to the total cost required to effect this mode of operation. The system also has the disadvantage that the time to complete a data request in this manner is unnecessarily long.
U.S. Pat. Ser. No. 4,540,849 to Oliver discloses a system which allows a much simpler unit to be located at the customer's meter. The central computer utilizes special test trunk equipment in the telephone central office to send a special interrogation signal which is detected by a meter interface unit located near the customer's meter. This type of test trunk access provides a communication path to the customer's premise without ringing the telephone. In order for the meter interface unit to operate without interfering with normal calls, the device draws a relatively low level of power from the telephone line. The difficulty with this disclosure is that it does not deal with the fact that U.S. federal regulation limits the power that may be drawn from inactive customer lines to an extremely low level considered beyond the state of the art. The current regulation, 47 C.F.R. 68.312, requires that a device attached to the phone lines have an effective on-hook DC resistance between tip and ring of greater than 5 megohms. This corresponds to only 9.6 microamps of available supply current at 48 volts.
A patent issued to Rosenfeld, U.S. Pat. Ser. No. 4,644,103, discloses an attempt to satisfy the extremely low limits which the regulations place on drawing current from the telephone lines. In this patent a periodic circuit draws current from the telephone lines at a permissible rate to charge a capacitor over a relatively long time. For a minor portion of each cycle this stored charge is used to briefly operate the tone detection circuit. If a tone is sensed, the unit activates for meter reading. The difficulty with this design is that the signal tone must be three to four seconds long in order to be certain that the tone detection circuit will be operable during a portion of the tone duration. This three to four second response time results in lower throughput of readings by the central computer, as well as increased usage fees by the telephone company when tariffs are based on the time central office equipment is in use.
Consequently, there was still a need for an inexpensively constructed, telephone line powered tone detection device which allows meter reading equipment at individual meters to be efficiently accessed while complying with the law and causing minimal interference with normal customer telephone use. No prior art tone detection circuits were available which would operate within the low current supply requirements while avoiding the problems of noise susceptability and temperature dependence.